The best sexual innuendo is to say something that can be construed that way, but not blatantly, and not acknowledge it or bust a smile or laugh, then when she's brings up any response to the sexual comment, shoot her a started or surprised look like.. what kinda chick is this? or blame her for mind i nthe gutter. "Yeah, I think women think about sex more than men."
I had a really good example of this but can't think of it now.. the girl was totally blamed for thinking dirty.. even though I set her up.
Some cruder comments that touch the subject but don't really put you on the spot:
A chick really working soemthing over or beating on it or making a ruckus "leave your frustrations i nthe bedroom" (telling her she's sexually frustrated or needs sexed more..)
comenting their clothes/ hair or makeup by saying they're trying to call attention to those features. "If you didn't where those pants I wouldn't be looking (when blatantly staring), but then, surely that wasn't your plan."
Staring at her cleavage without trying to hide it, then say (especially if they call you on it) "Don't blame me, you dressed you." (among girl friends, or playful-potentials.. this aint a good cold pickup...unless you're a PUA guru.)
"Gee, why would a nice girl like you be calling attention to her mouth?" ( Lipstick, liprings/ tongue rings,)
"Your car sure has a big backseat."
"I'm sure it's an unplanned coincidence the lettering's across your ass" (those coed/workout, everything else pants)
etc etc. The less you know them, the more subtle or denying about what you meant you should be.. as it wamrs up, you can play more, but in general, until it's pretty locked-up I wouldn't blatantly be talking sex... even 18 y/o's know what both of you are thinking during banter.